The present invention relates to a rare earth bonded magnet and a rare earth-iron-boron type magnet alloy, and more particularly, to a rare earth bonded magnet which has a high residual magnetic flux density (Br), a large intrinsic coercive force (iHc) and a large maximum energy product ((BH)max) in spite of a low rare earth element content, a rare earth-iron-boron type magnet alloy which has a residual magnetic flux density (Br) as high as not less than 10 kG, an intrinsic coercive force (iHc) as large as not less than 3.5 kOe and a large maximum energy product ((BH)max) and which has an excellent rust preventability, a process for producing the rare earth-iron-boron type magnet alloy, and a bonded magnet produced from such a rare earth-iron-boron type magnet alloy.
Bonded magnets which are advantageous in that they can be produced in any shape and have a high dimensional accuracy, etc., have conventionally been used in various fields such as electric appliances and automobile parts. With a recent development of miniaturized and lightweight electric appliances and automobile parts, bonded magnets used therefor have been strongly required to be miniaturized.
For this purpose, magnets have been strongly required to have a high residual magnetic flux density (Br), a large intrinsic coercive force (iHc) and, as a result, a large maximum energy product ((BH)max).
Bonded magnets using magneto plumbite type ferrite (referred to as `ferrite bonded magnet` hereinunder) which have conventionally been used for bonded magnets have an excellent rust preventability because ferrite is an oxide. In addition, since the ferrite bonded magnets are produced from a cheap material such as oxides of barium and strontium and an iron oxide, the ferrite bonded magnets are economical and are, therefore, widely used.
As to the magnetic characteristics of a general ferrite bonded magnet, however, the residual magnetic flux density (Br) is about 2 to 3 kG, the intrinsic coercive force (iHc) is about 2 to 3 kOe, and the maximum energy product ((BH)max) is about 1.6 to 2.3 MGOe.
Conventionally, rare earth bonded magnets represented by Nd-type isotropic compression-molded magnets are widely used for electric appliances in the form of magnets for motors. Especially, the rare earth bonded magnets are widely used for appliances mounted on computers such as hard disk drives (HDD) and CD-ROMs, peripheral devices of computers such as printers and scanners, and portable communication devices such as pocket telephones.
With the miniaturization and lightweight of the electric appliances and automobile parts, it is strongly demanded to provide a magnet which has higher magnetic characteristics and which is more economical. For example, as compared with the Nd-type isotropic bonded magnet, a rare earth sintered magnet (Nd-type or Sm-type) and a Sm-type anisotropic bonded magnet have a large maximum energy product ((BH)max), but are inferior in economical and they are therefore hardly used for electric appliances in the form of magnets for the above-mentioned motors.
As the magnet powder as a material of Nd-type isotropic compression-molded bonded magnets, the magnet powder MQP (trade name, produced by MQI Corp.) developed by GM Corp. in USA is only one at present which is supplied on an industrial scale. Especially, the magnet powder of MQP-B grade is chiefly used. The general composition of the MQP-B powder is Nd.sub.12 Fe.sub.76.5 Co.sub.5.5 B.sub.6 in the vicinity of the stoichiometeric composition of an Nd.sub.2 Fe.sub.14 B.sub.1 type crystal structure. As to the nominal magnetic characteristics, the residual magnetic flux density (Br) is 8.2 kG, the intrinsic coercive force (iHc) is 9.0 kOe, the maximum energy product ((BH)max) is 12.0 MGOe. As to the magnetic characteristics of a compression-molded bonded magnet (MQI-B10) produced from this magnet powder, the residual magnetic flux density (Br) is 6.9 kG, the intrinsic coercive force (iHc) is 9.0 kOe, the maximum energy product ((BH)max) is 10.0 MGOe.
Japanese Patent Application Laid-Open (KOKAI) No. 8-124730 (1996) describes a rare earth resin magnet having an intrinsic coercive force as low as 4 to 10 kOe, which is produced by mixing a rapidly chilled powder having a composition in the vicinity of the stoichiometeric composition of Nd.sub.2 Fe.sub.14 B.sub.1 in which Nd is 12.+-.0.5 atm % and an intrinsic coercive force iHc of 10 kOe and an exchange-spring magnet constituted by a soft magnetic phase and a hard magnetic phase in which crystal grain size is controlled to 20 to 50 nm, and solidifying the obtained mixture with a resin. However, the object of the invention of Japanese Patent Application Laid-Open (KOKAI) No. 8-124730 (1996) is to provide a rare earth resin magnet having an excellent multipolar-magnetizability. Therefore, the invention of Japanese Patent Application Laid-Open (KOKAI) No. 8-124730 (1996) is aimed at lowering the intrinsic coercive force by mixing powders. The magnetic characteristics described in the Examples do not exceed those of the MQI-B10.
As described above, in spite of an increasing demand for magnets which have a high magnetic force and which can be economically produced, no magnet which satisfies the demand from the point of view of both performance and economy has ever been proposed.
There is no end to a demand for a higher performance and a lower price of a magnet. To meet such a demand, rare earth-iron-boron type alloys for exchange-spring magnets using Nd as a rare earth element have been earnestly developed and some of them have already been put to practical use.
An exchange-spring magnet exhibits a magnetic spring phenomenon by the exchange interaction of iron or an iron compound and an Nd.sub.2 Fe.sub.14 B.sub.1 type tetragonal compound. Those magnets are characterized in a low rare earth element content and a high residual magnetic flux density (Br), and have a high possibility of being excellent on a cost/performance basis.
A rare earth-iron-boron type alloy for exchange-spring magnets containing less than 10 atm % of a rare earth element such as Nd, has a high potential in magnetic characteristics as compared with a rare earth-iron-boron type magnet alloy containing about 10 to 15 atm % of a rare earth element such as Nd which is in the vicinity of the stoichiometeric composition, e.g., commercially available "MQP" (trade name)" developed by General Motors. Since it is possible to reduce the amount of expensive rare earth element used, this alloy is economically advantageous.
The rare earth-iron-boron type alloy for exchange-spring magnets containing less than 10 atm % of a rare earth element such as Nd is divided into two systems as the soft magnetic phase: one is a system containing .alpha.Fe or bccFe, and the other is a system containing Fe.sub.3 B or Fe.sub.2 B. The former generally has a residual magnetic flux density (Br) as high as 10 to 13 kG but the intrinsic coercive force (iHc) thereof is as low as 3.5 kOe at most. The latter generally has a comparatively high intrinsic coercive force (iHc) such as 3.5 to 7.7 kOe, but the residual magnetic flux density (Br) thereof is as low as less than 10 kG, which is higher than that of "MQP" but lower than that of the former .alpha.Fe system.
In the field of small-sized motors for which bonded magnets produced from a rare earth-iron-boron type magnet alloy is mainly used, bonded magnets are required to have well-balanced residual magnetic flux density (Br) and an intrinsic coercive force (iHc) from the point of view of miniaturized of motors and magnetic stability of the magnets used therefor. That is, magnets are strongly required to have a residual magnetic flux density (Br) of not less than 10 kG and an intrinsic coercive force (iHc) of not less than 3.5 kOe.
On the other hand, an alloy containing rare earth elements in an Nd system is defective in that it is easily oxidized in the air and is likely to produce an oxide, so that the rust preventability is poor. Since bonded magnets produced from an alloy containing a rare earth element in an Nd system have a poor rust preventability, they are usually subjected to rust preventive coating-treatment such as dipping, spread coating or electro deposition using a resin and metal plating.
If the rust preventability of an alloy containing a rare earth element in an Nd system is enhanced, it may be possible to simplify or omit the rust preventive coating step for the surfaces of bonded magnets even for the above-described use. In some uses of general-purpose motors, there is a possibility of omitting the rust preventive coating step. Therefore, the enhancement of the rust preventability of a rare earth-iron-boron type magnet alloy is strongly demanded.
As described above, there is also a strong demand for the economical production of a rare earth-iron-boron type magnet alloy which has a high residual magnetic flux density (Br), a comparatively large intrinsic coercive force (iHc), and as a result, a large maximum energy product ((BH)max), and an excellent rust preventability.
In conventional quenched permanent magnet materials which contain Fe as the main ingredient (less than 91 atm %) and further contain at least one rare earth element (R) and boron (B), a permanent magnet material is known which comprises less than 10 area % of a soft magnetic residual amorphous phase based on the total alloy structure and a crystalline phase as the balance which is substantially produced by heat-treatment and which contains an R--Fe--B type hard magnetic compound (Japanese Patent Application Laid-Open (KOKAI) No. 8-162312 (1996)).
Although the economical production of a rare earth-iron-boron type magnet alloy which has a high residual magnetic flux density (Br), a comparatively large intrinsic coercive force (iHc), and as a result, a large maximum energy product ((BH)max), and an excellent rust preventability is now in the strongest demand, no magnet ever produced has such properties.
In the rare earth-iron-boron type magnet alloy described in Japanese Patent Application Laid-Open (KOKAKI) No. 8-162312 (1996)), the intrinsic coercive force (iHc) is as low as less than 3 kOe and the residual magnetic flux density (Br) is as low as less than 10 kG, as is clear from Table 5 in the specification in which the residual magnetic flux density (Br) is about 0.62 to 0.97 T (equivalent to 6.2 to 9.7 kG), the intrinsic coercive force (iHc) is about 0.16 to 0.21 MA/m (equivalent to 1.25 to 2.6 kOe), the maximum energy product ((BH)max) is about 19.7 to 72.0 kJ/m.sup.3 (equivalent to 2.5 to 9.0 MGOe).
The rare earth-iron-boron type magnet alloys described in Examples 2 to 4 of Japanese Patent Application Laid-Open (KOKAI) No. 8-162312 (1969) are bulk bodies obtained by pulverizing a quenched ribbon and extruding the pulverized particles under a vacuum. The bulk bodies are, therefore, different from a rare earth-iron-boron type magnet alloy as a raw material for bonded magnets in its configuration.
The theoretical energy product of a permanent magnet is generally represented by: (BH)max=0.25.times.Br.sup.2. Therefore, in order to obtain a bonded magnet having a higher energy product than MQI-B10, it is essential to use magnetic powder having a higher Br. In this case, with respect to intrinsic coercive force (iHc), if the condition: iHc&gt;0.5.times.Br is satisfied, the squareness is not impaired and the energy product is not lowered. It is, therefore, possible to use magnetic powder having a lower intrinsic coercive force (iHc) than that of MQI-B10 so long as the above-described condition is satisfied.
The present inventors have hit upon an idea of mixing two types of magnetic powders (A) and (B) in order to improve the magnetic characteristics of a bonded magnet. As a result of various experiments using an exchange-spring magnet powder (B) in an .alpha.Fe--NdFeB system and a magnetic powder (A) which contains a smaller amount of rare earth element than the MQP-B magnetic powder and is therefore economically advantageous, and which has a lower residual magnetic flux density (Br) than the magnetic powder (B) but an intrinsic coercive force (iHc) of not less than 7 kOe which is higher than that of the magnetic powder (B), by the combination of specific magnetic powder (A) and specific exchange-spring magnet powder (B), it has been found that a bonded magnet having the magnetic characteristics of a residual magnetic flux density (Br) of not less than 8 kG, an intrinsic coercive force (iHc) of not less than 5 kOe and a large maximum energy product ((BH)max) of not less than 11 MGOe has been firstly produced. Namely, it has been found that this bonded magnet is more excellent in residual magnetic flux density (Br) and maximum energy product ((BH)max) than the bonded magnet MQI-B10 in spite of a lower intrinsic coercive force (iHc), and that it is more excellent from the point of view of economy. On the basis of this finding, the present invention has been achieved.